1. Field of the Invention
The present invention is related to a splittable film and a production method thereof, e.g. a base film for a bag to pack foods, medicines, industrial products and so on.
2. Description of the Related Art
In recent years, films such as linear chain low density polyethylene (L-LDPE) have been widely used as a sealable base film for bags to pack foods, medicines and so on. However, this L-LDPE film was safe to pack contents because the seal strength was excellent, but its split resistance was too large for cutting the film straight along a predetermined direction when opening up a bag. In consideration of these problems, various inventions have been proposed to give the film the features of ease in splitting and ease in cutting straight.
Examples: (a) a laminated film using a uniaxial oriented film for a middle layer film (Japanese Patent Application Publication No. 58-38302 bulletin, Japanese Patent Application Publication No. 55-31725 bulletin); (b) a laminated film that is easier to split because of minute blemish in the surface thereof; (c) a (laminated) film with an opening tape attached to an opening part; and so on.
In the laminated film (a) mentioned above, because one layer is added as the middle layer film, the material cost and the processing cost to laminate films on top of one another increase. A uniaxial oriented film is used as a middle layer film but its use does not contribute strength. In the laminated film (b), even if the ease of opening can be improved by the minute blemishes on the film, the ease in cutting the film straight is controlled by the feature of the base material itself. Even-if the blemishes on the surface are few, there is a fear of decreasing the strength of the film. As for the film (c), because the tape for the opening is additionally used, the cost becomes higher, which may cause a decline in the productivity.
Also, conventionally, a plastic film was heat-treated to fix molecular orientation after the film was biaxially oriented by the tubular method for the purpose of obtaining stability in the dimension. In the heat-treatment, when a film of tubular nylon 6 folded flatly is heat-treated by the tenter method, the upper and lower films melt and stick to each other and thereafter it is impossible to separate them into two sheets. Thus, the film processed by means of prior art was unusable as a product. These problems are conspicuously seen when a plastic film is a crystalline thermoplastic resin film like a polyamide. Formerly, to solve the technical problems, one improved tenter method (Japanese Patent Application Publication No. 46-15439 bulletin) was proposed, wherein the film that was folded flat after being biaxially oriented in a tubular situation is separated into two films by cutting both edges in the direction of the width, they are introduced into the tenter and both edges of each film are held with clips to keep a certain distance between each film. Incidentally, adopting the tubular method, where the tubular film formed into a bubble shape by compression of air is heat-treated, can solve the problem of sticking together of the films due to heat. However, though in the heat treatment based on the above-mentioned improved tenter method, the films did not stick to each other, a bowing phenomenon (a phenomenon where a delay in the orientation at the central part occurs compared with both side parts) conspicuously occurred. Though lowering a heat treatment temperature naturally decreased the bowing ratio, it brought about a new problem of increasing the shrinkage percentage in the heat treatment of the boil, retort and so on.
The purpose of the present invention is to provide an oriented film that is easy to split and features excellent splitability, ease in cutting the film straight and enough impact strength and also to provide a production method for the oriented splittable film to obtain these features effectively.